Field of the Invention
The present invention relates to a phase difference control component comprising a liquid crystal material, which has excellent symmetry with respect to the angle of visibility and is optically compensable, and a process for producing the same, and a liquid crystal display device comprising said phase difference control component.
Background Art
Color liquid crystal displays (hereinafter often referred to as “LCDs”) have features such as thin shape, light weight, low power consumption, and flickerless, and the market of color liquid crystal displays for PCs including notebook computers has been rapidly grown. In recent years, regarding displays for PCs, there is an increasing demand for desktop monitors which are larger in size than notebook computers. Further, LCDs have become utilized in PCs, as well as in TVs for which CRTs have hitherto been mainly utilized.
A small angle of visibility is a problem inherent in LCDs. This problem is caused by light leakage from pixels, which should originally display black, when LCDs are viewed in an oblique direction. Due to this light leakage, the inversion of contrast takes place, and proper display becomes impossible. As a result, the angle of visibility is lowered.
In order to solve the above problem, LCDs with a high angle of visibility using a phase difference film have been proposed. The phase difference film is also used in combination with a linear polarization plate for creating various polarized states. For example, circularly polarized light can generally be constituted by a combination of a linearly polarizing plate with a λ/4 (quarter-wave) phase difference plate.
In these phase difference films, transparent polymeric films such as polycarbonate films subjected to stretching treatment such as monoaxial stretching have hitherto been used. In addition to these films, films formed by aligning and fixing a liquid crystal material having refractive index anisotropy while imparting given regularity can also be used. The phase difference film using a liquid crystal material can constitute, through alignment control of liquid crystal molecules, positive and negative A plates, positive and negative C plates, and a phase difference layer with hybrid alignment in which the aligned state is continuously varied (see, for example, Japanese Patent Laid-Open No. 11-153712 A1).
An aligning film subjected to alignment treatment is necessary for horizontally aligning liquid crystal molecules against a base material in a monoaxial direction. In general, polyimide films subjected to rubbing treatment are generally used as the aligning film. When liquid crystal molecules are aligned in a rubbing direction on the aligning film subjected to rubbing treatment, a pretilt angle (an angle of liquid crystal molecules to the base material) is created so that the end of the liquid crystal molecules are lifted in the rubbing direction relative to the base material. This pretilt angle plays an important role in regulating the liquid crystal molecules so that, upon voltage application, the liquid crystal molecules are lifted unidirectionally. Therefore, the presence of a pretilt angle in the liquid crystal molecules is indispensable, and, at the same time, the pretilt angle should be controlled.
When a fixed liquid crystal material is used as the phase difference control component, however, the presence of a pretilt angle in liquid crystal molecules causes a change in the level of phase difference to become asymmetrical with respect to the vertical direction upon a change in angle of visibility in a direction other than the phase advance axis. In particular, when the angle of visibility is varied in an optical axis direction of liquid crystal molecules, the asymmetry of a change in level of the phase difference is most significant.
Specifically, in the formation of an aligning film by the conventional rubbing method, as shown in FIG. 2, an aligning film 22 is coated on a base material 21 (FIG. 2(a)). The aligning film is rubbed, for example, by a rubbing roller 23 or the like (FIG. 2(b)). Next, a liquid crystal material is coated on the aligning film subjected to rubbing treatment. Molecular liquid crystals 24 make a pretilt angle θ with the direction of rotation of the rubbing roller 23 for alignment (FIG. 2(c)).
For this reason, when the level of phase difference is measured by varying the observation angle in a direction other than the phase advance axis, the level of the phase difference is asymmetrical with respect to 0 (zero) degree (vertical direction). In particular, when the level of phase difference is measured by varying the observation angle in the direction of the slow phase axis, the asymmetry of the level of the phase difference is most significant. For this reason, a liquid crystal display device to which a phase difference control component formed of a liquid crystal material with a pretilt angle θ has been applied, suffers from a problem that, except for a change in observation angle in the phase advance axis, display images are different.
Since the pretilt angle also propagates in the thickness-wise direction of the film, when the phase difference layer comprises a plurality of liquid crystal layers stacked on top of each other, the alignment at the interface of the liquid crystal layers is adversely affected.
For oblique compensation of angle of visibility, the necessary phase difference level should be calculated for design of the phase difference control component. When the level of the symmetry of the angle of visibility is low, a desired phase difference level can be realized only in any one direction and a phase difference control component, which can realize a satisfactorily large angle of visibility, cannot be provided.
In large-size liquid crystal televisions, due to properties of angle of visibility within a display face attributable to the large area, the symmetry of the angle of visibility poses a severe problem.